The present invention generally relates to reliably sensing misfire in a cylinder of an engine, and more particularly relates to methods and apparatus for also accurately indicating repeated misfires to an operator of the engine and for identifying which of the cylinders is misfiring.
Governmental agencies require modern internal combustion engines being used for a variety of purposes to provide warnings to an operator of the engine when a cylinder or cylinders thereof repeatedly misfire. The misfire can be caused by either an improper fuel-to-air mixture due to a faulty fuel injector or by an inadequate amount of spark energy caused by a faulty spark plug, for instance. Such warning indication is usually in the form of a light that is activated in response to a warning signal from a misfire sensing system or apparatus associated with the engine. The misfire sensing apparatus is also required to facilitate engine repairs by recording and providing to a service technician the identity of the cylinder or cylinders that initiated the warning.
A misfire results in incomplete combustion of the fuel and air mixture in a cylinder which causes a fuel rich gas to be outputted by the engines of automobiles for example, into catalytic converters. Precious metals utilized in such converters facilitate further chemical burning of the fuel rich gas resulting from the misfire so that these gases are consequently cleaned before being exhausted from the converters into the ambient atmosphere. Such converters can properly process only a limited amount of such incompletely combusted gas before being destroyed. The repetition of a nearly complete lack of combustion by a misfiring cylinder of an engine experiencing either a heavy load or a high rotational speed can output an air-to-fuel mixture so rich in fuel that the mixture will destroy a catalytic converter in a short time. A misfire also results in a reduction in the torque being delivered by the misfiring cylinder to a crankshaft being driven by the cylinder. Thus it important that the conditions causing misfiring be quickly corrected in response to the warning indication to help maintain engine power, preserve air quality and to avoid costly repairs.
Some prior art misfire sensing systems utilize gears driven by the crankshafts to indicate misfires. The gear includes precisely formed teeth for alternately affecting the flow of magnetic flux through a magnetic sensor as the crankshaft turns the gear. Consequently, the sensor provides an electrical pulse for each selected amount of crank angle of the crankshaft. The misfire sensing system measures the time periods between successive pulses generated by the magnetic sensor for each cylinder combustion event. The reduced torque associated with misfire causes the time period corresponding to the misfiring cylinder combustion event to increase relative to the cylinder combustion event time periods of the non-misfiring cylinders. The time periods of the cylinder combustion events for each of the cylinders are compared with each other. It is desired that this comparison help determine the occurrence of the misfire and help identify which cylinder or cylinders are misfiring by identifying which cylinder or cylinders have combustion event time periods that are long enough to indicate a misfire. The misfire warning signal is generated in response to the occurrence of a number of misfires sufficient to verify that a misfiring problem exists. Also, known misfire sensing systems learn the Crank Angle Sensing Error (CASE) due to differences between the individual teeth of the gear to thereby increase the signal-to-noise ratios of such misfire indication signals.
The rotational speed of an engine is measured in Revolutions per Minute (RPM). The RPM of the engine and thus the cylinder combustion event time periods used for the foregoing misfire determinations by the previously described prior art misfire sensing apparatus are subject to significant changes due to various factors other than misfire. Such factors include transient or temporary influences on the engine from the drive train of a vehicle driving on a rough road, for instance. Moreover it is also difficult for some prior art misfire systems to determine misfire when the engine is at idle. This is because the engine controller adjusts the engine parameters over wide ranges in an attempt to maintain a constant RPM at idle even though the load on the engine changes because of the activation and deactivation of various systems such as air conditioning or electric windows. Such varying loads affect the cylinder combustion event time periods. Detection of misfire by some prior art systems becomes more troublesome as the number of cylinders of an engine increases because each cylinder combustion event becomes shorter for a given engine RPM as the number of cylinders increases. The shorter time periods tends to create erroneous results in prior art systems having low signal-to-noise ratios because there is less available time to ascertain each misfire.
Furthermore, some prior art misfire sensing apparatus have too much sensitivity, which results in erroneous misfire warnings. Alternatively, other prior art systems have too little sensitivity, which results in no misfire warning even though the engine is misfiring. The erroneous warnings or the lack of valid warnings undesirably cases an unacceptable number of newly manufactured engines to not pass test procedures relating to detection of misfiring.
In view of foregoing, it should be appreciated that there is a need to provide improved methods and apparatus for robustly detecting cylinder misfire, providing a warning signal in response to a sufficient number of misfires and identifying the misfiring cylinder or cylinders. There is also a need to provide misfire sensing systems that have high misfire signal-to-noise ratios so that the sensitivity tolerance can be kept within manageable limits. Moreover, it is desirable to not provide false warning alerts to the operators of such engines. Such methods and apparatus are required to operate in an improved manner at engine idle and over both large and transient variations of RPM. It is also desirable for such improvements to be accomplished by using information already being provided by the engine controller for other purposes rather than by providing additional expensive hardware. Moreover, it is advantageous for such methods and apparatus to be suitable for use with engines having a relatively high numbers of cylinders. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent brief summary, detailed description, appended claims and abstract taken in conjunction with the accompanying drawings and this background of the invention.
In accordance with an exemplary embodiment of the present invention, methods and apparatus are provided which operate with an engine having at least one cylinder for sensing a misfire and providing a misfire indication signal. The apparatus comprises an electronic engine controller that calculates an expected time period value for a cylinder event during which fuel is combusted in a cylinder. The actual time period of the cylinder combustion event is also measured to provide a measured time period value. The difference between the expected time period value and the measured time period value is calculated to provide a difference time period value. The difference time period value is compared to a predetermined threshold time. The misfire indication signal is provided in response to the difference time period value exceeding the predetermined threshold time.